Electrostatic and electrophotographic printing involves developing a latent electrostatic image with charged toner particles loaded onto an imaging drum and transferring them onto a substrate or a print substrate, particularly in the form of sheets or in the form of a continuous conveyor belt. As an example, in four-color printing, four latent images in the four-color separations (cyan, magenta, yellow, and black) are transferred to the substrate successively and in register on top of one other. In particular, the finished single color or multicolored latent image is then fused onto the substrate by a fusing device. This customarily takes place by a heatable fusing roller, which is rolled onto the toner image. The toner is heated up above its glass transition temperature, and thus melted, and simultaneously incorporated under pressurization into the substrate to which it is fused after it has been cooled. Adjacent toner particles are thereby combined, which finally form a polymer layer on the substrate.
A problem can occur with the described procedure, if a greater number of printing processes are to be carried out within a specific period of time, such that the method should be accelerated. Then the fusing process may actually prove to be the speed-limiting factor of the printing process, because it cannot be linearly accelerated. If the fusing process needs to be accelerated, it may be thought that the temperature of the fusing roller needs to be increased and/or that the fusing nip area between the fusing roller and a counter-pressure roller needs to be enlarged in the substrate-transfer direction.
However, an increase in the temperature leads to a reduced service life of the fusing roller, particularly the sheathing or cladding. Furthermore, during fusing with a fusing roller, silicone oil is used as a separating agent, to prevent the toner from sticking to the fusing roller and damaging subsequent printing processes. This oil must be frequently topped up and its use is increased, whereby there is also the danger of it sticking to the conveying devices, soiling them and tracking it further, so that this oil may also damage subsequent printing processes.
If the fusing nip area is to be enlarged, this can be accomplished in two ways. The pressure between the fusing roller and the counter-pressure roller can be increased and, as a result, a larger flattened area is created, or a fusing roller with a larger diameter can be used. Increasing the pressure may in turn reduce the service life of the fusing roller, particularly the sheathing, and this can lead to substrate damages, in particular, to the crumbling of the substrate. If the diameter of the fusing roller is enlarged, this may easily lead to jamming of the substrate. As a result, the construction costs and dimensions become problematic.
The object of the invention is thus based, in particular for an increased speed, on reliably conducting and yet improving the fusing process with a method and a device of the above-mentioned type using a trustworthy, proven technique, preferably utilizing a fusing roller.